Preparation of organic acid esters of cellulose



Patented July 15, i952 PREPARATIQN OF ORGANIC ACID ESTERS OF CELLULOSE Mervin E. Martin, Cumberland, Md., assignor to Celanese Corporation of America, New York, N. Y., a corporation of Delaware No Drawing. Application December 30, 1949, Serial No. 136,174

Claims. (Cl. 260229) This invention relates to the preparation of organic acid esters of cellulose, and relates more particularly to an improved process for the preparation of organic acid esters of cellulose in a continuous manner. I

In the preparation of organic acid esters of cellulose by the esterification of a cellulosic material, it is customary to pretreat the cellulosic material before esterification to increase its reactivity and thereby reduce the total esterification time. This pretreatment may, for example, be effected by mixing the cellulosic material for a period of several hours with a lower aliphatic acid in either liquid or vapor form. Following the pretreatment, the cellulosic material is en-' tered into an esterification mixture comprising an organic acid anhydride, a diluent or a solvent for the ester being formed and' an esterification catalyst. V

The reaction which takes place when the pretreated cellulosic material is entered-into the esterification mixture is strongly exothermic and tends to raise the temperature of said mixture. If the temperature is permitted to rise unchecked, it will cause a degradation of the cellulose molecule, resulting in the production of an organic acid ester of cellulose of extremely low viscosity that is not suited for the preparation of filaments and films. To prevent such a degradation of the cellulose molecule, the esterification mixture is cooled initially to a low temperature and, in addition, external cooling is applied to the reaction mixture with the object of further controlling the temperature thereof. Usually, the heat generated during the esterification is permitted to cause a gradual and controlled increase in the temperature of the reaction mixture over a period of'several hours to a peak-of 35 to 50 C. At this stage of the esterification, a part of the cellulosic material is not completely esterified. Accordingly, the reaction mixture is held atthe peak temperature for an additional period'of several hours until allof the cellulosic material is fully esterified. r I

Since both the pretreatment andthe esterification require several hours, it has hitherto not been practical to'carry on these steps infa continuous manner; 'I nstead,- it has beennec'essary to'carry on both the pretreatment and esterification on abatchbasisfrequiringa large-number of individual reactors,-- thus'invo'l'vi'ng afgreat deal of laborand a large capitaloutlay.

' It isan important object of this invention to provide a process for preparing organic acid estersof cellulose which will be free from the 2 foregoing and other disadvantages of the processes hitherto employed for this purpose and which will be especially simple and eflicient in operation.

A further object of this invention is the provision of a process for preparing organic acid esters of cellulose which may be carried out in a continuous manner.

Other objects of this invention will be apparent from the following detailed description and claims.

According to this invention, the cellulosic material is conditioned sothat it has a moisture content of between about 5 and 8% by weight, or preferably between about 6.5 and 8.0% by weight, and the conditioned cellulosic material is then subjected to a pretreatment in two stages. In the first stage of the pretreatment, the cellulosic material is treated with a lower aliphatic acid. Then, after an interval of at least about 2 minutes, the cellulosic material is treated for a,

period not exceeding about15 minutes with an additional quantity of a lower aliphatic acid containing from about 5 -to 15 parts by weight, for each parts by weight of the cellulosic material, of an esterification catalyst.

I have found that by means of this novel process, the time necessary for the pretreatment may be reduced materially from that previously necessary. In addition. the reactivity of the cellulosic material is greatly increased so that the esterification of the pretreated cellulosic material proceeds to completion more rapidly than heretofore. The reactivity of the pretreated cellulosic material is so great that, despite external cooling, the heat liberated by the exothermic esterification reaction raises the temperature of the esterification mixture to values high enough to cause a degradation of the cellulose molecule. To prevent such an excessive increase in temperature during esterification, the pretreated cellulosic material is cooled so as to cause at least a portion of the lower aliphatic acid present therein to freeze.

The pretreated cellulosic material, containing the frozen lower aliphatic acid, is then brought into contact with an organic acid anhydride which has beencooled to a" temperature below about 10 C. The temperature of the mixture so formed is permittedtorise to a value of between about 15 and 25 .C., and .is held within said temperature rangeof between about 15 and 25 C. for a period of from aboutl tolO minutes.-

The temperature of the esterific'ati'onmixtureis then permitted to rise further .toja valueoifrom fication mixture. J

, 3 about 30 to 65 0., with the application of external heat, if necessary, and it is held within said temperature range until the esterification is complete. I have found that the major portion of the cellulosic material is esterified without suffering any appreciable loss in viscosity during the initial stages of th'e'esterification, i. e. while the temperature of the reaction mixture is below about 25 C. When the temperature is raised, the relatively small proportion of unesterified cellulosic material still present is esterified rapidly without in any way impairing the properties of the organic acid ester of cellulosebeing produced. i v I In accordance with my invention, the time required for both the pretreatment and esterifieation of the cellulosic material be reduced to as little as minutes, or even less. Accordingly, the pretreatment and esterification may be readily carried out in a continuous manner.

'While my invention will be described in connection with the preparation of cellulose'acetate, it is to be unde'rstoodthat it is also applicable to the preparation of other organic acid esters of cellulose such as, for example, cellulose .propionate, cellulose acetate-.p-ropionate, cellulose butyrate, cellulose acetate-butyrate and the like.

The process of the present invention may be applied to any suitable cellulo'sicmaterial such as, for example, cotton linters or wood pulps or the like from which the lignin, pentosan, resins and similar constituents have been removed, as for example, sulfite pulp, fsulfate pulp or soda pulp. In addition, there may be employed 'cellulosic material derived from other. sources such as bamboo, reeds, jute, esparto, bagasse, straws such aslinseed and hempseed straw, grasses, peanut hulls, bark, etc. The 'cel'lulosic material may be employed in the form of a mass of loose fibers, or

' in the form of a sheet of matted cellulosic fibers,

which sheet may be forwarded continuously while the several treating media are'applied thereto.

' ing periods are not objectionable since they do not affect unfavorably the properties of the pretreated cellulosic material. The lower aliphatic acid may be applied to the cellulosic material at room temperature or at a temperature above or below room Before the cellulosic material is pretreated, it

must be conditioned so that it hasa moisture content of between about 15 and 8% byweight, or

preferably between about 6.5 and 8.0% by weight. This conditioning maybe readily. efiecte'dby spraying the required quantity of water onto the cellulosic material, "or. by passing the cellulosic material through a chamber maintained at a high humidity by the injection therein of steam or water. I have found that if the moisture content of the cellulo's'ic material drops below 7 the value specified,'the pretreatment and esterie fication will both require a considerably longer period of time. addition of water to the lower aliphatic acid employed for pretreating thecellulosic material is not equivalent to the presence of moisture in the cellulosic material'in that it does not reduce the time required for pretreatment and esterification. If, on the other hand, the moisturecontent of the cellulosic material rises above thevalue specified, the excess water will hydrolyze a por r 'tion of the organic acid anhydride employed for the e'sterification, necessitating theuseof larger ,eiquantities of anhydride and liberating a great deal ofhe atfwhich willtend tocause an undesirable increase inthetemperature ofthe'esteri Any .of the megawatts scar matting acetic, propionic and g-butyric acids, ormixture thereof, may be employed durin'g the first;pre'- treating stage in concentrations ;rang ing from 80 to 100% by weight, .thegremainder being about temperature, but is preferably applied at a temperature of from about 30 to 40 C. for best results.

After the first pretreating stage and before the second pretreating stage, an interval of at least about, minutes ispermitted to elapse, during which time the lower aliphatic acidenters into the fibers of the eellulosic material- This interval may be kept to a minimum by bringing the cellulosic material to a temperature ranging from about 40 to 80% C. In a continuous process, the

necessary interval of time between the first and second pretreatingstages may be obtained by applying the pretreating agents at spaced points and forwarding the cellulosic material from one point to another at a given speed.

The first pretreating stage i followed by the second pretreating stage wherein the cellulosic material is treated for a period not exceeding about 15 minutes with an additional quantityof a lower aliphatic acid containing fromaboutfi to 15 parts by Weight of; an esterification catalyst for each 100 parts by weight of the cellulosic material. If a longerpretreating period is employed, there is the danger that the high concentratien of esterification catalyst will degrade the cellulose molecule and thereby cause an excessivereduc- 95 to 100% by'weight, theremainder-being water,

I have also found'that the and in quantities rangin ,zrromabout '200 'toGQO parts by weightfor each 100 parts by weight of the cellulosic material. 1 The lower aliphatica-cid containing the esterification catalyst-may be applied to the cellulosic material at room temperature or at a temperature above or below room temperature.

Any of the known esterifieaticn catalysts may be employed for :pretreating the .CBlllllOSiC material in accordance-with this invention, -but'it is preferred to employ sulfuricacid-ior this purpose since it produces a cellulosiematerial havingthe hi h st reactivity in the shortest period of --time.

; h p t ea n agents ma -b "app iedth cellulosic material-in any desired manner. qF or example, they may be sprayed or rolled onto the 'cellu losic material, or when the cellulosiqmate- "rial s in 'sheet form, they may. be applied -as-a continuous; film from aperforated or slotted egg;-

duit-extending thefwidthvf the sheet.

- Puring the second p retreating stage,--the;cellu;- lo i te ia is cool d to m ra u e l w he ee s ro nt-o thelowe al h ia d e ployed for-thepretreatrrient aIid-isheld at this temperature until at least-about 30% of theslower aliphatic acid-is frozen. 3, When acetic acid is emp oyed for t e pretreatmentpt e lulosic m t a corresponding reduction in the quantities of said materials employed during the second pretreating stage; The temperature of the esterification mixture is permitted to rise to a value of between about and C. and is held within,

said temperature range for a period of from about 1 to 10 minutes. The temperature of the esterification mixture is then permitted to rise further to a value of from about to 65 0., with the application of external heat, if necessary, and

it is held within said temperature range of from about 30 to"65 C.'unti1 the esterification is complete, which normally takes about 2 to 15 minutes.. The temperature of the esterification mixture may be permitted to rise continuously during the esterification, provided that it remains withinthe range of values set forth above for the time specified.

After the esterification is complete, a sufficient quantity of water is added to the esterification mixture to convert any remaining organic acid anhydride to .the corresponding acid, following which the cellulose ester, preferably after the addition of a further quantity of water, is permitted to hydrolyze or ripen to impart the desired solubility, characteristics thereto. The cellulose ester is then precipitated from the esterification mixture, washed to free it from acids and other impurities, stabilized, if necessary, washed again and finallydried.

The following example is given to illustrate this invention further.

Example A mass of willowed cotton linters is passed continuously at a rate of 5 parts by weight per minute through a cylindrical mixer equipped with stirrer blades and having a length to give a total retention time of the cotton linters in the mixer of 2 minutes. A sufficient quantity of water is sprayed into the mixer to bring the moisture content of the cotton linters to 7.5% by weight. The so-conditioned cotton linters are entered continuously into a second cylindrical mixer equipped with stirrer blades and with a jacket through which a, heating medium is passed to raise the temperature of the cotton linters to 0. As the conditioned cotton linters enter the second cylindrical mixer, they are sprayed with acetic acid in an amount of 35 parts by weight of acid for each 100 parts by weight of cotton linters. The cotton linters remain in the second cylindrical mixer for 2 minutes and are then entered continuously into a third cylindrical mixer equipped with stirrer blades and a jacket to maintainthe temperature of the pretreated cotton linters at 35 C. As the cotton linters enter the third cylindrical mixer, they are sprayed with a mixture of acetic acid and sulfuric acid in an amount of 1'72 parts by weight of acetic acid and 14 parts by weight of sulfuric acid for each 100 parts by weight of cotton linters. The cotton linters are held in the third cylindrical mixer for 2 minutes with a cooling medium passed through the jacket to reduce temperature of the pretreated cotton linters to 15C. andfreeze at least aportion of the acetic acid. 'From the third mixer, the pretreated cotton lintersare entered continuously into a first acetylizer equipped with stirrer blades and with a jacket through which a. cooling medium is passed to control the temperature of the esterification mixture to a maximum of 25 .C'. .Simultaneously, there is entered, into the first acetylizer a mixture of acetic anhydride. and acetic acid cooled to .--15 C. in an amount of 249 parts .by weight of acetic anhydride and 166 parts by weight of acetic acid for each 100 parts by weight of cotton linters. After 2 minutes in the first acetylizer, the cotton linters are entered continuously into a second acetylizer equipped with stirrers and with a jacket through which a heating medium is'passed to bring the temperature of the esterification mixture to 45 C. The esterification mixture remains in the second acetylizer for 6 minutes and is discharged therefrom in the form of a clear viscous dope that may be neutralized, hydrolyzed, precipitated and washed in a manner well known inthe art.

It is to be understood that the foregoing detailed description is given merely by way of illustration and that many variations may be made therein without departing from the spirit of my invention.

Having described my invention, what I desire to secure by Letters Patent is:

1. In a process for the preparation of organic acid esters of cellulose from cellulosic material, the steps which comprise subjecting the cellulosic material to a multi-stage pretreatment prior to esterification, one stage of said pretreatment comprising the addition to the cellulosic material of a lower aliphatic acid and another stage of the pretreatment comprising the addition to the cellulosic material containing the lower aliphatic acid of a further quantity of a lower aliphatic acid containing from about 5 to 15 parts by weight of an esterification catalyst for each 100 parts by weight of cellulosic material without displacing any of the lower aliphatic acid contained in the cellulosic material, freezing at least a portion of the lower aliphatic acid contained in the cellulosic material, and esterifying the cellulosic material containing the frozen lower aliphatic acid and all the pretreating agents within a period of less than about 15 minutes after the addition thereto of the lower aliphatic acid containing the esterification catalyst.

2. Process according to claim 1, wherein at least about 30% by weight of the lower aliphatic acid in the cellulosic material is frozen.

3. Process according to claim 1, wherein the esterification is efiected by first holding the esterification mixture at a temperature of from about 15 to 25 0., and then holding said" mixture at a temperature of from about 30 to 65-C. until the esterification is complete.

4. Process according to claim 1, wherein from about 15 to parts by weight of acetic acid for each parts by weight of cellulosic material are employed during the initial stage of the pretreatment, from about 200 to 600 parts by weight of acetic acid for each 100 parts by weight of cellulosic material are employed during the subsequent stage of the pretreatment, and sulfuric acid is employed as the esterification catalyst.

5. Process according to claim 4 wherein at least about 30% of the acetic acid in the cellulosic mathe 75 terial is frozen and wherein the esterification is efieetejdby first holdingjhe sterification.mixtu at a temperature 01' -from about 15 to 25 C. for a period of from about 1 to minutes, and then holding, said mixture at a temperature of from about 3.0 to 65 C; until theesterification' is com plete.

6. In a process: for the preparation of cellulosic acetate from cellulosic. material, the steps which comprise subjecting the cellulosic material to, a multi-stage pretreatment priorto acetylation, one

, stage of said pretreatmentcomprising the addition to the cellulosic materialof a lower aliphatic acid and another stage of the pretreatment comprising the additionto the cellulosic material containing the lower aliphatic acid of; a further quantity ofa loweraliphatic acid containing from about Etc parts by weight of anacetylation catalyst for each 100 partsb weight-of cellulosic material Without displacing any of the lower aliphatic acid contained in the cellulosic material, freezing at least a portion of the lower aliphatic acid contained in the cellulosio material, and 'acetylating the cellulosic material containin t frozen lower aliphatioacid and all the pretreating agents within aperiod of less than about 1'5 minutes after the addition thereto of the lower aliphatic acid containing the acetylationcatalyst.

7. 'Process according to claim .6, wherein at least about 30% by weight of the lower aliphatic acid in the cellulosic material is frozen. V

8. Process accordingfto claim ,6, wherein the acetylation is effected by first holding the-acetylation mixture at a temperature of from about 15 to C.,'and then holding said mixture at a temperaturelof from about to C. until the acetylation is complete. 7

9. Process according toclaim 6, wherein from about15 to parts by weight of acetic acid for i The following each parts by: wei htoficellu os c mat r a are employed during the initial sta e 10: the pretreatment, ,fromlabput 200 110.600 parts by Wei ht of acetic acid for each :100parts by weight of c.ellu losic material are employed during the subsequentstage of the pretreatment, and sulfuric acid is employed as they acetylation catalyst.

10; Process according "to claim '9 wherein at least iabout30% of the acetic ,acidin the cellulosic material is frozen. and wherem the acetylation is effected by firstholding theac'etyIatiQn mixture at a temperature of. fromiabout 15 to 253C. for a period offromiabout. Ito 1.0 minutes, and then holding said mixture atua; temperature .01" from about '30 to 65C: until the acetylation is coinplete.

' i, MERE/IN .E. MARTIN.

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6. IN A PROCESS FOR THE PREPARATION OF CELLULOSIC ACETATE FROM CELLULOSIC MATERIAL, THE STEPS WHICH COMPRISES SUBJECTING THE CELLULOSIC MATERIAL TO A MULTI-STAGE PRETREATMENT PRIOR TO ACETYLATION, ONE STAGE OF SAID PRETREATMENT COMPRISING THE ADDITION TO THE CELLULOSIC MATERIAL OF A LOWER ALIPHATIC ACID AND ANOTHER STAGE OF THE PRETREATMENT COMPRISING THE ADDITION TO THE CELLULOSIC MATERIAL CONTAINING THE LOWER ALIPHATIC ACID OF A FURTHER QUANTITY OF A LOWER ALIPHATIC ACID CONTAINING FROM ABOUT 5 TO 15 PARTS BY WEIGHT OF AN ACETYLATION CATALYST FOR EACH 100 PARTS BY WEIGHT OF CELLULOSIC MATERIAL WITHOUT DISPLACING ANY OF THE LOWER ALIPHATIC ACID CONTAINED IN THE CELLULOSIC MATERIAL, FREEZING AT LEAST A PORTION OF THE LOWER ALIPHATIC ACID CONTAINED IN THE CELLULOSIC MATERIAL, AND ACETYLATING THE CELLULOSIC MATERIAL CONTAINING THE FROZEN LOWER ALIPHATIC ACID AND ALL THE PRETREATING AGENTS WITHIN A PERIOD OF LESS THAN ABOUT 15 MINUTES AFTER THE ADDITION OF THE LOWER ALIPHATIC ACID CONTAINING THE ACETYLATION CATALYST. 